The transmission of an image representative signal such as a television signal through a transmission channel may subject the signal to a variety of disturbances which may alter desired parameters of the signal. In particular, transmission channel characteristics may cause the frequency and phase parameters of the signal to deviate from a desired "flat" characteristic. This result may be produced by both terrestrial broadcast and cable channels, for example.
To compensate for such deviations, i.e., to re-acquire a desired flat frequency and phase response of the signal, a signal equalizer may be used at a receiver. The use of such an equalizer is particularly important with the advent of high definition television systems. In digital systems multiple-tap FIR filters are employed by the equalizer apparatus. For removing signal distortions an equalizer may compare a test signal, which is incorporated in the received signal, with a programmed ideal or desired version of the test signal to configure a filter at the receiver so as to remove or compensate for the irregularities in the received television signal. The test signal may be in the form of a pseudorandom number (PRN) sequence as described, for example, in U.S. Pat. No. 5,065,242--Dieterich in the context of deghosting apparatus for a television receiver.
It is herein recognized that an accurate determination of the received signal's average or DC level is important for fast Fourier transform (FFT) ratio equalization. Failure to properly determine the DC level can lead to improperly set DC gain, which can produce what appear to be image "smudges" in a displayed image. Illustratively, this effect will cause a display of a person situated against a flat field background to be surrounded by a "glow" or "shadow". Depending upon equalizer design, this effect can be centered on the displayed image or offset to one side, and is basically due to a very low frequency signal component produced by an offset in all tap values of the equalizer filter resulting from mis-measured DC characteristics.